Orientation-induced crystallization of isotactic polypropylene

In polymer processing operations, the molten polymer chains are frequently subjected to shear or/and elongation flow fields, which will produce molecular chain orientation of the melt. This leads to the orientation-induced crystallization has been an important subject in the field of polymer physics. Systematic studies indicated that the chain orientation influences the crystallization kinetics, the final morphology as well as the polymorphic behavior of the polymers. In this article, the effects of preorientation on the crystallization of isotactic polypropylene (iPP) concerning the above mentioned aspects have been reviewed. In particular, the formation mechanism of orientation-induced β-iPP crystallization has been discussed according to the recent experimental results. It is suggested that the local order of the macromolecular chain segments in the melt is most important for β-nucleation of iPP. The formation of β-iPP nuclei may be restricted in a certain chain orientation window of the iPP melts. Chain orientation outside of this window results in the formation of α-iPP.